Merge my api-work development branch. It includes:

* Fix for slurmctld behaviour when environment is NULL on batch script launch
* Move client (srun) IO code into the API in preparation for adding
  an API call for launching a normal parallel job step.
* Rename SLURM_CRED_SIGLEN back to SLURM_IO_KEY_SIZE, but put it in a better
  header.